Loudspeaker diaphragm motion generates acoustic energy in front of and behind the loudspeaker. The acoustic energy in front provides the expected loudspeaker acoustic output. The acoustic energy in the back is usually confined so that it does not interfere with the loudspeaker acoustic output in the front, but can provide a measure of the loudspeaker acoustic output. In handheld devices, such as smart phones and cell phones, for example, loudspeakers are usually implemented with a sealed back cavity design. That is, the acoustic energy generated in the back of the loudspeaker is confined within a sealed cavity. In this case, an acoustic pressure measurement in the back cavity serves as a measure of the loudspeaker acoustic output. The loudspeaker acoustic output serves as a reference for many purposes. The loudspeaker acoustic output is used as a reference in digital signal processing (DSP) algorithms such as echo cancellation.